Flip Chip Interconnection Structure with Bump on Partial Pad and Method Thereof

ABSTRACT

A semiconductor package includes a semiconductor die with a plurality of solder bumps formed on bump pads. A substrate has a plurality of contact pads each with an exposed sidewall. A solder resist is disposed opening over at least a portion of each contact pad. The solder bumps are reflowed to metallurgically and electrically connect to the contact pads. Each contact pad is sized according to a design rule defined by SRO+2*SRR−2X, where SRO is the solder resist opening, SRR is a solder registration for the manufacturing process, and X is a function of a thickness of the exposed sidewall of the contact pad. The value of X ranges from 5 to 20 microns. The solder bump wets the exposed sidewall of the contact pad and substantially fills an area adjacent to the exposed sidewall. The contact pad can be made circular, rectangular, or donut-shaped.

FIELD OF THE INVENTION

The present invention relates in general to semiconductor packages and,more particularly, to a flip chip interconnect structure having a bumpon a partial pad.

BACKGROUND OF THE INVENTION

Semiconductor devices are found in many products in the fields ofentertainment, communications, networks, computers, and householdmarkets. Semiconductor devices are also found in military, aviation,automotive, industrial controllers, and office equipment. Thesemiconductor devices perform a variety of electrical functionsnecessary for each of these applications.

The manufacture of semiconductor devices involves formation of a waferhaving a plurality of die. Each semiconductor die contains hundreds orthousands of transistors and other active and passive devices performinga variety of electrical functions. For a given wafer, each die from thewafer typically performs the same electrical function. Front-endmanufacturing generally refers to formation of the semiconductor deviceson the wafer. The finished wafer has an active side containing thetransistors and other active and passive components. Back-endmanufacturing refers to cutting or singulating the finished wafer intothe individual die and then packaging the die for structural support andenvironmental isolation.

One goal of semiconductor manufacturing is to produce a package suitablefor faster, reliable, smaller, and higher-density integrated circuits(IC) at lower cost. Flip chip packages or wafer level packages (WLP) areideally suited for ICs demanding high speed, high density, and greaterpin count. Flip chip style packaging involves mounting the active sideof the die facedown toward a chip carrier substrate or printed circuitboard (PCB). The electrical and mechanical interconnect between theactive devices on the die and conduction tracks on the carrier substrateis achieved through a solder bump structure comprising a large number ofconductive solder bumps or balls. The solder bumps are formed by areflow process applied to solder material deposited on metal contactpads which are disposed on the semiconductor substrate. The solder bumpsare then soldered to the carrier substrate. The flip chip semiconductorpackage provides a short electrical conduction path from the activedevices on the die to the carrier substrate in order to reduce signalpropagation, lower capacitance, and achieve overall better circuitperformance.

FIG. 1 illustrates a portion of flip chip 10 with a solder bump 12formed on metal contact pad 14. The solder bump 12 is thenmetallurgically and electrically connected to metal contact pad 15 onsubstrate 16 using a solder reflow process. To connect solder bump 12and contact pad 15, a solder resist or mask opening 18 is disposed overa surface of the substrate to confine the solder reflow to the physicalboundaries of contact pad 15, see FIG. 2 a. Due to manufacturingalignment tolerances as to the relative position between contact pad 15and solder resist opening 18, contact pad 15 is made substantiallylarger than solder resist opening 18 to ensure that the full metal areaof the contact pad is exposed, given the alignment tolerance of thesolder resist opening. In generally, the minimum size of contact pad 15is P_(min)=SRO+2*SRR, where SRO is the minimum solder resist openingrequired to ensure good metallurgical connection and SRR is the solderresist alignment tolerance, also known as solder registration. In oneexample, if solder resist opening 18 is 90 microns and the solder resistalignment tolerance is 25 microns, then, according to the known designrule, contact pad 15 is made 140 microns in diameter. Thus, under theknown design rule, and given the maximum manufacturing alignmenttolerance, the solder resist opening always falls within the contact padand leaves no voids or empty space around the pad, as shown in FIG. 2 b.

Unfortunately, the larger contact pad required to ensure that the solderresist opening always falls within the full metal area of the contactpad limits the metal signal trace or track routing density that can beachieved on the substrate. The larger contact pad necessarily reducestrace routing density as fewer traces can be placed between the contactpads. In addition, the larger contact pad translates to fewer contactspads per unit area of the substrate.

SUMMARY OF THE INVENTION

A need exists to minimize the contact pad size to increase trace routingdensity without impacting electrical functionality or manufacturingreliability. Accordingly, in one embodiment, the present invention is amethod of making a semiconductor package in a manufacturing processcomprising the steps of providing a semiconductor die with a pluralityof solder bumps formed on bump pads, providing a substrate having aplurality of contact pads each with an exposed sidewall, disposing asolder resist opening over at least a portion of each contact pad, andreflowing the solder bumps to metallurgically and electrically connectto the contact pads. Each contact pad is sized according to a designrule defined by SRO+2*SRR−2X, where SRO is the solder resist opening,SRR is a solder registration for the manufacturing process, and X is afunction of a thickness of the exposed sidewall of the contact pad.

In another embodiment, the present invention is a method of making asemiconductor device comprising the steps of providing a substratehaving a contact pad with an exposed sidewall, and disposing a solderresist opening over at least a portion of the contact pad. The contactpad is sized such that the solder resist opening extends over thesubstrate by a maximum distance of X which is a function of a thicknessof the exposed sidewall of the contact pad.

In another embodiment, the present invention is a method ofinterconnecting a semiconductor die to a substrate comprising the stepsof providing a semiconductor die with a solder bump formed on a bumppad, providing a substrate having a contact pad with an exposedsidewall, and disposing a solder resist opening over at least a portionof the contact pad. The solder resist opening extends over a portion ofthe substrate by a maximum distance of X which is a function of athickness of the exposed sidewall of the contact pad. The solder bump isreflowed to electrically connect to the contact pad. The solder bumpextends down the exposed sidewall of the contact pad.

In another embodiment, the present invention is a semiconductor packagecomprising a semiconductor die with a solder bump formed on a bump pad.A substrate has a contact pad with an exposed sidewall. The solder bumpis electrically connected to the contact pad. The contact pad is sizedaccording to a design rule defined by SRO+2*SRR−2X, where SRO is asolder resist opening used to form the solder bump, SRR is a solderregistration for the manufacturing process, and X is a function of athickness of the exposed sidewall of the contact pad.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conventional solder bump interconnect to a contact pad on aflip chip; FIGS. 2 a-2 b illustrate a conventional alignment design rulefor a solder resist opening over the contact pad;

FIG. 3 is a flip chip semiconductor device with bumps providingelectrical interconnect between an active area of the die and a chipcarrier substrate;

FIGS. 4 a-4 d illustrate a contact pad reduced in size by a 2 X reduceddesign rule which allows for misalignment between the contact pad andsolder resist opening; and

FIGS. 5 a-5 d illustrate alternate shapes for the contact pad subject tothe 2 X reduced design rule.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention is described in one or more embodiments in thefollowing description with reference to the FIG, in which like numeralsrepresent the same or similar elements. While the invention is describedin terms of the best mode for achieving the invention's objectives, itwill be appreciated by those skilled in the art that it is intended tocover alternatives, modifications, and equivalents as may be includedwithin the spirit and scope of the invention as defined by the appendedclaims and their equivalents as supported by the following disclosureand drawings.

The manufacture of semiconductor devices involves formation of a waferhaving a plurality of die. Each die contains hundreds or thousands oftransistors and other active and passive devices performing one or moreelectrical functions. For a given wafer, each die from the wafertypically performs the same electrical function. Front-end manufacturinggenerally refers to formation of the semiconductor devices on the wafer.The finished wafer has an active side containing the transistors andother active and passive components. Back-end manufacturing refers tocutting or singulating the finished wafer into the individual die andthen packaging the die for structural support and/or environmentalisolation.

A semiconductor wafer generally includes an active surface havingsemiconductor devices disposed thereon, and a backside surface formedwith bulk semiconductor material, e.g., silicon. The active side surfacecontains a plurality of semiconductor die. The active surface is formedby a variety of semiconductor processes, including layering, patterning,doping, and heat treatment. In the layering process, semiconductormaterials are grown or deposited on the substrate by techniquesinvolving thermal oxidation, nitridation, chemical vapor deposition,evaporation, and sputtering. Photolithography involves the masking ofareas of the surface and etching away undesired material to formspecific structures. The doping process injects concentrations of dopantmaterial by thermal diffusion or ion implantation.

Flip chip semiconductor packages and wafer level packages (WLP) arecommonly used with integrated circuits (ICs) demanding high speed, highdensity, and greater pin count. Flip chip style semiconductor device orpackage 20 involves mounting an active area 22 of die 24 facedown towarda chip carrier substrate or printed circuit board (PCB) 26, as shown inFIG. 3. Active area 22 contains active and passive devices, conductivelayers, and dielectric layers according to the electrical design of thedie. The bump pads 34 are formed on active area 22 using an evaporation,electrolytic plating, electroless plating, screen printing, or othersuitable metal deposition process. Bump pads 34 connect to the activeand passive circuits by conduction tracks in active area 22. The contactpad 36 can be aluminum (A1), tin (Sn), nickel (Ni), gold (Au), silver(Ag), or copper (Cu). The electrical and mechanical interconnect isachieved through a solder bump structure 30. A solder material isdeposited on bump pads 34 using an evaporation, electrolytic plating,electroless plating, ball drop, or screen printing process with anymetal or electrically conductive material, e.g., Sn, lead (Pb), Ni, Au,Ag, Cu, bismuthinite (Bi), and alloys thereof. For example, the soldermaterial can be eutectic Sn/Pb, high lead, or lead free. The soldermaterial is reflowed to form solder bumps or balls 32. The solder bumps32 are electrically and mechanically connected to contact pads orinterconnect sites 36 on carrier substrate 26 by a solder reflowprocess. The flip chip semiconductor device provides a short electricalconduction path from the active devices on die 24 to conduction trackson carrier substrate 26 in order to reduce signal propagation, lowercapacitance, and achieve overall better circuit performance.

Further detail of the solder bump connection to the substrate contactpad is shown in FIGS. 4 a-4 d. Solder bump 32 is formed on bump pad 34in active area 22 of semiconductor die 24, as described above. Metalcontact pad 36 is formed on substrate 26 using an evaporation,electrolytic plating, electroless plating, screen printing, or othersuitable metal deposition process. The contact pad 36 can be A1, Cu, Sn,Ni, Au, or Ag. Contact pad 36 has a sidewall 42 with an exposedthickness T. FIG. 4 a, a solder resist or mask opening 45 is disposedover at least a portion of contact pad 36. Depending on its alignment,the solder resist opening 45 may extend over a portion of substrate 26,as shown in FIG. 4 c. To electrically and metallurgically connect solderbump 32 to contact pad 36, the solder bump is reflowed through solderresist opening 45 onto the contact pad, as shown in FIGS. 4 b and 4 d.

In another embodiment, solder material can be deposited through thesolder resist opening 45 directly onto contact pad 36 on substrate 26using an evaporation, electrolytic plating, electroless plating, balldrop, or screen printing process with any metal or electricallyconductive material, e.g., Sn, Pb, Ni, Au, Ag, Cu, Bi, and alloysthereof. The solder material is reflowed to form a solder bump, asdescribed above.

The manufacturing process for semiconductor device 20 employs a newdesign rule including a solder registration (SRR) which defines analignment tolerance for the solder resist opening. Under the new designrule, the solder resist opening need not necessarily be aligned withinthe contact pad, but rather can be offset or misaligned with respect tothe physical area of the contact pad due to manufacturing alignmenttolerances.

FIGS. 4 a and 4 b show the ideal case where solder resist opening 45 issubstantially aligned to a center region of contact pad 36. In practice,contact pad 36 may be misaligned with respect to solder resist opening45 during the manufacturing process. For example, in FIG. 4 c, thesolder registration in combination with the smaller contact pad resultsin solder resist opening 45 extending outside the footprint of contactpad 36 by a distance of X. That is, solder resist opening 45 is offsetor misaligned to the physical area of contact pad 36. The new designrule allows for this misalignment between contact pad 36 on substrate 26and solder resist opening 45. During the wetting process, the surfacetension of the solder flux material causes the solder bump 32 to reflowacross and adhere to surface 46 of contact pad 36. In cases wherecontact pad 36 is misaligned with solder resist opening 45, i.e. aportion of the contact pad extends outside the solder resist opening,the reflow of solder bump 32 also overlaps and wets sidewall 42 ofcontact pad 36 to substantially fill the area adjacent to sidewall 42,as seen in FIG. 4 d. The solder bump 32 leaves no void or empty spacearound contact pad 36, including sidewall 42. In either case, under thepresent design rule, solder bump 32 makes good metallurgical andelectrical connection to contact pad 36.

As a feature of the present invention, the contact pad 36 is madesmaller, relative to solder resist opening 45, as compared to dimensionsand design rules found in the prior art. In general, the contact pad 36is sized according to the following design rule:

P _(size) =SRO+2*SRR−2X   (1)

-   -   where: SRO is solder resist opening, SRR is solder registration        or manufacturing alignment tolerance, and X is a design rule        defining the amount by which the solder bump can overlap the        edge and sidewall of the contact pad

The new design rule reduces the size of contact pad 36 by 2X inaccordance with equation (1). The value of X is a function of the fluxmaterial and thickness of contact pad 36. In general, the value of Xranges from one to two times the thickness of the exposed sidewall ofthe contact pad. The exposed portion of contact pad 36 (T) is typically5-20 microns in thickness. The value of X can increase with fluxeffectiveness. Accordingly, given a 90-micron solder resist opening anda 25-micron solder resist alignment tolerance, contact pad 36 can bemade 120 microns, assuming a design rule with X=10 microns. In anotherexample, given a 90-micron solder resist opening and a 25-micron solderresist alignment tolerance, contact pad 36 can be made 60 microns,assuming a design rule with X=40 microns. In practice, the contact padis typically made size equal to or smaller than the solder resistopening. In other cases, the contact pad can be larger than the solderresist opening.

The 2X reduced design rule of equation (1) provides smaller contact padswhich provides for more contact pads per unit area of the substrate. Inaddition, the smaller contact pads provide more space for signal tracerouting between the contact pads, thereby increasing trace routingdensity. The 2X design rule maintains reliability of the metallurgicalconnection and electrical functionality of the semiconductor device. Thereflowed solder material adhering to sidewall 42 increases themetallurgical integrity and reliability of interconnection by providingmore solder contact area. Since the solder material wets to sidewall 42,little or no voids are formed between the solder bump and sidewall 42.The area of substrate 26 immediately around contact pad 36 iselectrically isolated to avoid shorting the solder bump to adjacenttraces or devices.

FIG. 5 a shows contact pad 50 with a circular shape that is made thesame size or slightly smaller than solder resist or mask opening 52. Inthis case, the overlap or wetting of the sidewall of the contact padoccurs completely around the circumference of the contact pad.Alternatively, the contact pad can be made larger than the solder resistopening.

In FIG. 5 b, contact pad 60 is shown with a generally circular shape andnarrow trace lines extending from the pad. The trace lines aresubstantially narrower than solder resist opening 62. The overlap orwetting of the sidewall of the contact pad occurs substantially aroundthe circumference of the contact pad.

In FIG. 5 c, contact pad 70 is made rectangular to further reduce theexposed volume when solder resist opening 72 is misaligned.

In FIG. 5 d, a donut-shaped contact pad 80 provides for wetting of thesolder material through solder resist opening 82 on the inner and outersidewalls of the contact pad. Contact pad 80 reduces the wettablesurface area of the pad and provides higher standoff after solder jointis formed.

In summary, given the alignment tolerance for the solder resist opening,the solder bump will necessary overlap the contact pad in at least someof the manufactured semiconductor devices. By employing the 2X reduceddesign rule that reduces the contact pad size according to equation (1)and allows the solder material to overlap and wet sidewall 42, thecontact pads can be made smaller in size which provides for placement ofmore signal traces between the contact pads. The signal trace routingdensity correspondingly increases. In addition, the smaller contact padstranslate to more pads per unit area of the substrate. From the priorart design rule discussed in the background, which resulted in a140-micron contact pad with a 90-micron solder resist opening, the 2Xreduced design rule substantially reduces the contact pad by 2X.

While one or more embodiments of the present invention have beenillustrated in detail, the skilled artisan will appreciate thatmodifications and adaptations to those embodiments may be made withoutdeparting from the scope of the present invention as set forth in thefollowing claims.

1. A method of making a semiconductor package in a manufacturing process, comprising: providing a semiconductor die with a plurality of solder bumps formed on bump pads; providing a substrate having a plurality of contact pads each with an exposed sidewall; disposing a solder resist opening over at least a portion of each contact pad; and reflowing the solder bumps to metallurgically and electrically connect to the contact pads; wherein each contact pad is sized according to a design rule defined by SRO+2*SRR−2X, where SRO is the solder resist opening, SRR is a solder registration for the manufacturing process, and X is a function of a thickness of the exposed sidewall of the contact pad.
 2. The method of claim 1, wherein a value of X ranges from 5 to 20 microns.
 3. The method of claim 1, wherein the contact pad is equal in size to the solder resist opening.
 4. The method of claim 1, wherein the contact pad is smaller in size than the solder resist opening.
 5. The method of claim 1, wherein the contact pad is larger in size than the solder resist opening.
 6. The method of claim 1, wherein the solder bump substantially fills an area adjacent to the exposed sidewall of the contact pad.
 7. The method of claim 1, wherein a shape of the contact pad is selected from the group consisting of circular, rectangular, and donut-shaped.
 8. A method of making a semiconductor device, comprising: providing a substrate having a contact pad with an exposed sidewall; and disposing a solder resist opening over at least a portion of the contact pad; wherein the contact pad is sized such that the solder resist opening extends over the substrate by a maximum distance of X which is a function of a thickness of the exposed sidewall of the contact pad.
 9. The method of claim 8, wherein the contact pad is sized according to a design rule defined by SRO+2*SRR−2X, where SRO is the solder resist opening, SRR is a solder registration for the manufacturing process, and X is a function of the thickness of the exposed sidewall of the contact pad.
 10. The method of claim 8, further including: providing a semiconductor die with a solder bump formed on a bump pad; and reflowing the solder bump to electrically connect to the contact pad.
 11. The method of claim 8, further including forming a solder bump through the solder resist opening over the contact pad.
 12. The method of claim 8, wherein a value of X ranges from 5 to 20 microns.
 13. The method of claim 8, wherein the contact pad is equal in size to the solder resist opening.
 14. The method of claim 8, wherein the contact pad is smaller or larger in size than the solder resist opening.
 15. A method of interconnecting a semiconductor die to a substrate, comprising: providing a semiconductor die with a solder bump formed on a bump pad; providing a substrate having a contact pad with an exposed sidewall; disposing a solder resist opening over at least a portion of the contact pad, the solder resist opening extending over a portion of the substrate by a maximum distance of X which is a function of a thickness of the exposed sidewall of the contact pad; and reflowing the solder bump to electrically connect to the contact pad, the solder bump extending down the exposed sidewall of the contact pad.
 16. The method of claim 15, wherein the contact pad is sized according to a design rule defined by SRO+2*SRR−2X, where SRO is the solder resist opening, SRR is a solder registration for the manufacturing process, and X is a function of the thickness of the exposed sidewall of the contact pad.
 17. The method of claim 15, wherein a value of X ranges from 5 to 20 microns.
 18. The method of claim 15, wherein the contact pad is equal in size to the solder resist opening.
 19. The method of claim 15, wherein the contact pad is smaller or larger in size than the solder resist opening. 20-25. (canceled) 